Pneumatic ground transportation system

ABSTRACT

A gravity-vacuum ground transportation system in which a vehicle is propelled as a free piston through ducts from station to station along the route of the system, each duct having an entrance valve at its end at a first station, an exit valve at its end at the next station, and a pressure regulator valve in communication with a common manifold, and in which the pressure regulator valve and the opening and closing of the entrance and exit valves are controlled to control the passage of the vehicle from station to station in a manner such that the passenger carrying portion of the vehicle is immersed in atmospheric pressure air at all times.

United States Patent Edwards [151 3,656,436 [451 Apr. 17, 1972 [54] PNEUMATIC GROUND TRANSPORTATION SYSTEM [72] Inventor: Lawrence K. Edwards, 301 Santa Lita Avenue, Palo Alto, Calif. 94301 [22] Filed: Feb. 5, 1970 [21] Appl.No.: 8,863

Primary Examiner-Arthur L. La Point Assistant Examiner-D. W. Keen Attorney-Keenig, Senniger, Powers and Leavitt [57] ABSTRACT A gravity-vacuum ground transportation system in which a vehicle is propelled as a free piston through ducts from station to station along the route of the system, each duct having an entrance valve at its end at a first station, an exit valve at its end at the next station, and a pressure regulator valve in communication with a common manifold, and in which the pressure regulator valve and the opening and closing of the entrance and exit valves are controlled to control the passage of the vehicle from station to station in a manner such that the mospheric pressure air at all times.

10 Claims, 18 Drawing Figures PRE SSURE IN ATMOSPHERES PATENTEIJIPII I8 I972 3,656,436

SHEET u 0F 5 PRESSURE BEHIND VEHICLE 31 POSITION OF CENTER OF VEHICLE 5? m PRESSURE AHEAD OF VEHICLE ld I! I P a. I!) O ,2

u] [I '3 In I!) Ld (I (l POSITION OF CENTER OF VEHICLE 52 BACKGROUND OF THE INVENTION This invention relates to ground transportation systems, and more particularly to pneumatic systems of this class.

The invention is in the field of gravity-vacuum transit systems, such as shown in L. K. Edwards US. Pat. No. 3,404,638, issued Oct. 8, 1968 and U.S. Pat. No. 3,438,337, issued Apr. 15, 1969. It involves a variant of these systems, particularly well suited for urban mass transportation or other short-stage systems as distinguished from intercity transportation.

SUMMARY OF THE INVENTION Among the several objects of this invention may be noted the provision of a gravity vacuum ground transportation system especially suitable for a transit system in which the maximum distance between stations is relatively small, e.g., 00 to 5,000. feet, and extremely high speeds are not required, and the provision of such a system which reduces construction, equipment and operating costs. In general, the system involves the propulsion of a vehicle as a free piston through a duct from a first to a second station. The stations are in com munication with the earths atmosphere. The duct has an entrance valve at its end at the first station and an exit valve at its end at the second station, and a pressure regulator valve for communication with a vacuum manifold that is common to several such ducts. The duct preferably slopes downward from the stations for gravity assist in accelerating a departing vehicle and decelerating and arriving vehicle. Operation starts with the vehicle in the first station, both valves closed, and the air pressure in the duct reduced to the appropriate level by means of the pressure regulator valve. The entrance valve is opened to initiate the trip of the vehicle from the first to the second station. On opening the entrance valve, the vehicle is propelled from the first station past the entrance valve into the duct, atmospheric pressure behind the vehicle acting to accelerate the vehicle through the duct, the vehicle acting to compress air ahead of the vehicle. When the pressure of air ahead of the vehicle generally reaches atmospheric pressure, the exit valve is opened and the vehicle then coasts forward. The entrance valve is closed behind the vehicle so that, as the vehicle travels forward, air behind the vehicle is expanded and its pressure drops, whereby the vehicle decelerates, bringing it to rest in the second station, and the exit valve then being closed behind the vehicle. A key feature which distinguishes the present invention from previous ones is the arrangement of the system to cause the exit valve to open before the entrance valve is closed, thereby avoiding reduced pressure alongside the vehicle. Other objects and features will, be in part apparent and in part pointed out hereinafter.

The invention accordingly comprises the system arrangement and methods of operation hereinafter described, the scope of the invention being indicated in the following claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a profile of a short-stage gravity-vacuum system operable in accordance with this invention;

FIG. IA is an enlarged transverse section on line 1A-IA of FIG. 1;

FIG. 2 is an enlarged fragment of FIG. 1 showing a vehicle in a first station of the system;

FIGS. 3-6 are views similar to FIG. 2 showing the vehicle at different stages in its trip from the first to a second station;

FIGS. 7A and 7B are charts of a typical pressure curve showing the air pressure ahead of and behind the vehicle in the operation of the system according to the instant invention;

FIG. 8 is a side elevation of a train of cars constituting a vehicle which may be operated in accordance with this invention;

FIG. 9 is a transverse section on line 9-9 of FIG. 8;

FIG. 10 is a transverse section on line 10-10 of FIG. 8;

FIG. 10A is a detail of a modification; and

FIGS. ll-15 are views similar to FIGS. 2-6 illustrating the operation of the system according to this invention with a vehicle longer than the duct between stations.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawing, first more particularly to FIG. 1, there is shown gravity-vacuum transportation system in accordance with this invention having a plurality of stations indicated at S1, S2, S3 and S4 in FIG. 1, each of which is in communication with the earths atmosphere. Extending from station S1 to station S2 is a duct D1. Extending from station S2 to station S3 is a duct D2. Extending from station S3 to station S4 is a duct D3. Each duct preferably slopes downward from each of the stations which it interconnects for gravity assist in accelerating a departing vehicle and decelerating an arriving vehicle. Each duct has an entrance valve V1 at its end at the first of the two stations which it interconnects and an exit valve V2 at its other end at the second of the two stations which it interconnects. As shown in FIG. 1A, there may be two ducts extending side by-side between successive stations for travel of vehicles in opposite directions. The second duct in FIG. 1A, which is the duct alongside duct D3, is designated D3a. The ducts alongside D1 and D2 are not shown. A vacuum manifold 1 extends generally the full length of the system. A vacuum pump 3 is connected to the manifold for evacuating it as indicated at 5. The manifold is connected to each of the ducts by ports 7 each having a remote-controlled pressure regulator valve 9 therein.

FIGS. 2-6 illustrate the mode of .operation for a trip of a vehicle 11 from the first station S1 to the second station S2 through the first duct D1. FIG. 2 shows the vehicle 11 in the station S1, the entrance valve V1 to duct D] from station S1 and the exit valve from duct D1 to station S2 both being closed. The section of the duct D1 between the closed valves V1 and V2 is evacuated via the respective port 7 and vacuum manifold 1 down to a predetermined prepressure (on the order of V4 to atmosphere) suitable for the trip. Preferably this is determined by weighing the vehicle in station 81 after the doors of the vehicle have been closed and it is ready to depart, and basing the prepressure on the weight of the train (the greater the weight, the lower the prepressure, and vice versa). The desired prepressure is obtained by suitable operation of the pressure regulator valve 9 in the port 7 which interconnects the duct D1 and the vacuum manifold l. The prepressure is also based on the length of the duct (which is, of course, a constant) as well as the weight of the vehicle (which is variable) and is made such as to assure acceleration of the vehicle at the desired rate.

The trip is initiated by opening the entrance valve V1. On opening this valve, the vehicle is propelled from the first station S1 into the duct D1 as shown in FIG. 3. Atmospheric pressure behind the vehicle acts to accelerate the vehicle through the duct (noting that there is lower pressure ahead of the vehicle) and since the duct slopes down from station S1, the vehicle is also accelerated by gravity.

As the vehicle progress through the duct D1 it acts to compress the air in the duct ahead of it. When the pressure of air ahead of the vehicle generally reaches atmospheric pressure, the exit valve V2 from duct D1 is opened, and the vehicle begins forcing air out of the duct. The air ahead and behind the vehicle is then at atmospheric pressure and the vehicle coasts (except for losses and a small effect of gravity) as shown in FIG. 4.

At a suitable time, the deceleration phase is begun by closing the entrance valve V1 behind the vehicle. The vehicle then expands the air in the duct Dl behind it, and the pressure of air behind the vehicle drops. With reduced pressure behind and atmospheric pressure ahead, the vehicle decelerates as it approaches the second station S2 as shown in FIG. 5, deceleration being assisted by gravity as the vehicle travels up the slope toward station S2.

With proper selection of the time that the deceleration phase begins, the vehicle 11 comes to rest in the second station S2 and the exit valve V2 closes behind it as shown in FIG. 6. The trip is now complete and the vehicle is in a position to continue on to the next station S3 once passengers have detrained and entrained.

FIG. 7A is a chart in which the solid-line curve C1 depicts the pressure behind the vehicle as it progresses from station S1 to station S2 and FIG. 7B is a chart in which the solid-line curve C2 depicts the pressure ahead of the vehicle as it progresses from station S1 to station S2. As shown by curve C1 in FIG. 7A, the pressure behind the vehicle is one atmosphere until the entrance valve is closed and then decreases to about one-half an atmosphere, and as shown by curve C2 the pressure ahead of the vehicle increases from about onehalf an atmosphere to an atmosphere until the exit valve opens, then remains constant at 1 atmosphere.

The use of the manifold 1 and pressure regulator valve 9 in the system enables adjustment of the prepressure in each duct before and during each trip of a vehicle therethrough. This makes it possible for every vehicle operated in the system to accelerate at the same rate even though their loaded weights may vary. The manifold also serves as a buffer between the duct and the pump so that conditions at the inlet to the pump are fairly steady. Energy may be stored in the manifold by the steady, low-power operation of the pump and delivered to the vehicles in high-power bursts. The nominal pressure in the manifold may be considerably less than the operating pressure of the ducts, thus allowing the storage of large amounts of energy.

In some circumstances it may be desirable to vary the mode of operation by removing additional air from the duct ahead of the vehicle during part of the acceleration phase (FIG. 3) via the respective pressure regulator valve 9 in such manner that the pressure ahead of the vehicle 11 and thus the pneumatic acceleration of the vehicle remain constant for an extended acceleration phase. Thus, compression of the air in the duct ahead of the vehicle does not begin until air removal is stopped. Sufficient time for compression must be allowed so that the drop in acceleration rate is not too rapid for passenger comfort. Depending on how much air is removed during the acceleration phase, it may be necessary to add air during the deceleration phase so that the pressure does not drop too low during expansion and thereby cause excessive deceleration. This variation in the mode of operation is shown by the dotted-line curves C3 and C4 in FIGS. 7A and 7B.

An entire system of this type with many stages can be operated from a single central pump installation. This avoids electrical power distribution to many points and minimizes the need for dispersed equipment. All maintenance of pumps takes place at the same location so that maintenance equipment need not portable. Any noise or uncleanliness associated with pumping equipment is concentrated at a single location.

The system is designed so that for any trip, the exit valve always opens before the entrance valve closes. This is accomplished by choosing a vehicle of such weight and length that over the range of vehicle weights caused by varying passenger loading the required prepressure will result in the pressure ahead of the vehicle rising to atmospheric pressure before it is necessary to close the entrance valve for deceleration. (This condition is always satisfied if the prepressure is greater than one-half atmosphere, provided that the stations are at the same elevation.) This feature results in one end of the vehicle always being exposed to atmospheric pressure and eliminates the requirement that the passenger compartment be pressure sealed. Relief valves can be provided at each end of the vehicle to insure that the internal air remains at atmospheric pressure at all times. The entrance and exit valves can be interlocked so that both cannot be closed while a vehicle is in the duct.

Since high speeds are not encountered in this system, it is practical to operate with higher prepressures than in highspeed systems, perhaps as high as three-quarters of an atmosphere. (The pressure of one-half atmosphere shown in FIGS. 7A and 7B is by way of illustration only and not by way of limitation.) Computer simulation has verified operation at pressures as high as 10 psia. This is possible because airflow losses at these speeds are not prohibitive. Higher prepressures permit the use of lighter, and hence smaller, vehicles, as well as reducing the structural requirements for the ducts, thus permitting the use of noncircular The ducts can be equipped with safety valves or pressure-relief diaphragms to admit air in the event of a failure of the pressure regulator valve resulting in the pressure dropping to an unsafe level within the tube.

As shown in FIGS. 8, 9 and 10, the vehicle 11 for the system preferably consists of a train of identical passenger cars 15 each having a door 17 for the entry and discharge of passengers. End cars of the train are specially designated 15a. Clearance between the end cars and the walls of the duct is made sufficiently small that leakage of air around an end car in the duct is minimal. If necessary, a seal 19 may be provided at these points. Since one end of the vehicle or train will always be at atmospheric pressure, that portion of the duct that the train occupies can also be kept at atmospheric pressure by providing passages 20 extending lengthwise of the end cars having check valves 21 therein. The same effect may be achieved by making the seals at 19 unidirectional. FIG. 10A shows such a unidirectional seal (for the left end car) comprising a flexible resilient conical sealing ring 19a adapted to flex in toward the periphery of the car under atmospheric pressure on its outside and to flex out into tight sealing engagement with the duct under atmospheric pressure on its inside. Thus, the passenger cars 15 do not travel through a partial vacuum and need not be pressure sealed. In addition, the passenger cars are never required to prevent inrush of air into the tube as they pass through the entrance and exit valves; therefore, it is not necessary that they conform to the outside dimensions of the end cars. They may even be open coaches if means is provided to prevent passengers from reaching out to come in contact with the duct wall, valves, etc. There is no requirement for pressure-tight joints between cars or pressure-tight flexible joints to allow the train to negotiate curves. These may be used, if desired, of course. However, because they are not required, the train can negotiate considerably tighter curves than previously described vacuum-tube vehicles. The car wheels are indicated at 23 and the seats in the passenger cars are indicated at 25.

Due to the action of the check valves 21, the forward end car acts as a piston in a cylinder when the air pressure behind the train is atmospheric and the air pressure ahead is less than atmospheric. The rearward end car acts as a piston in a cylinder when the air pressure ahead is atmospheric and the air pressure behind is less than atmospheric.

The vehicle may be either longer or shorter than the duct through which it travels. In either case, the operation according to the instant invention is the same, as described above. FIGS. 2-6 inclusive illustrate the operation with a vehicle 11 shorter than the tube or duct; and FIGS. 11-15 inclusive correspond to FIGS. 2-6 inclusive and illustrate the operation with a vehicle 11! longer than the tube. For example, FIGS. 2 and 11 show the vehicle before the trip; FIGS. 3 and 12 show the acceleration phase; FIGS. 4 and 13 show the coast phase; FIGS. 5 and 14 show the deceleration phase; and FIGS. 6 and 15 show the vehicle at the end of the trip. The only significant difference in operation of a vehicle of the type shown in FIGS. 11-15 inclusive is that there is no need to interlock the entrance and exit valves because the vehicle can never be entirely inside the duct.

In view of the above, it will be seen that several advantageous results are attained by the instant invention.

As various changes could be made in the above methods without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. The method of operating a ground transportation system in which a vehicle is propelled as a free'piston through a duct from a first station at one end of the duct to a second station at its other end, the duct having an entrance valve at said one end and an exit valve at its said other end adapted when closed to block off a section of the duct from valve-to-valve, each of said stations being in communication with the earth's at-- mosphere, comprising starting with the vehicle in the first station, both valves closed, and said section evacuated to a pressure below atmospheric pressure; opening the entrance valve for propulsion of the vehicle from the first station past the entrance valve into the duct, atmospheric pressure behind the vehicle acting to accelerate the vehicle through the duct, and the vehicle acting to compress air ahead of the vehicle; opening the exit valve when the pressure of air ahead of the vehicle generally reaches atmospheric pressure, the vehicle then coasting; closing the entrance valve behind the vehicle so that, as the vehicle travels forward, air behind the vehicle is expanded and its pressure drops, whereby the vehicle decelerates, bringing the vehicle to rest in the second station; and then closing the exit valve behind the vehicle; and wherein the vehicle is weighted after having been loaded in the first station before opening the entrance valve; and the pressure in the evacuated duct is adjusted, in accordance with the weight of the vehicle and the length of the duct, to a value'such as to provide the desired acceleration.

2. In a ground transportation system having a duct through which a vehicle is propelled as a free piston from a first station at one end of the duct to a second station at the other end of the duct, the duct having an entrance valve at its said one end and an exit valve at its said other end adapted when closed to block off a section of the duct from valve-to-valve, each of said stations being in communication with he earth s atmosphere, a manifold, a pump for evacuating the manifold, and a port having a pressure regulator valve therein interconnecting the manifold and the duct, said system having a plurality of stations greaterthan two with a duct between each two successive stations, each duct having an entrance valve at its said one end and an exit valve at the other, said manifold extending lengthwise of the system, there being a port with a pressure regulator valve therein interconnecting the manifold and each of the, ducts;

3. In a ground transportation system as set forth in claim 2, the length of the vehicle being less than the length of the duct between two successive stations.

4. In a ground transportation system as set forth in claim 2, the length of the vehicle being greater than the length of the duct between stations.

5. In a ground transportation system as set forth in claim 2, said system having two ducts extending side-by-side between each two successive stations, there being a port with a pres sure regulator valve therein interconnecting the manifold and each of the two ducts between successive stations.

6. In a ground transportation system having a duct through which a vehicle is propelled as a free piston from a first station at one end of the'duct to a second station at the other end of the duct, the duct having an entrance valve at its said one end and an exit valve at its said other end adapted when closed to block off a section of the duct from valve-to-valve, each of said stations being in communication with the earth s atmosphere, a manifold, a pump for evacuating the manifold, and a port having a pressure regulator valve therein interconnecting the manifold and the duct, said vehicle being a train which, adjacent its forward and rearward ends, has a sliding fit in the duct providing a seal which is sufficient to minimize leakage of air past its said ends, check valve means at the forward end of the train, and check valve means at the rearward end of the train, the forward check valve means permitting flow of air back to the train when the air pressure ahead of the train is atmospheric and the air pressure behind the train is less than atmospheric and checking the flow of air from behind the forward end of the train to the duct ahead of the train when the air pressure behind the train is atmospheric and the air pressure ahead of the tram is less than atmospheric, the

rearward check valve means permitting flow of air forward to the train from behind the train when the air pressure behind the train is atmospheric and the air pressure ahead of the train is less than atmospheric and checking the flow of air from ahead of the rearward end of the train to the duct behind the train when the air pressure ahead of the train is atmospheric and the air pressure behind the train is less than atmospheric.

7. In a ground transportation system as set forth in'claim 6, said train having forward and rearward end cars each having said sliding fit in the duct and passenger cars between the end cars, each check valve means comprising a passage through the respective end car and a check valve in said passage.

8. In a ground transportation system as set forth in claim 6, said check valve means being constituted by unidirectional seals adjacent the ends of the train.

9. In a ground transportation system having a duct through which a vehicle is propelled as a free piston from a first station at one end of the duct to a second station at the other end of the duct, the duct having an entrance valve at its said one end and an exit valve at its said other end adapted when closed to block off a section of the duct from valve-to-valve, each of said stations being in communication with the earth s atmosphere, means for evacuating said section of the duct to a selected prepressure before each trip of the vehicle therethrough, said prepressure being selected in accordance with the length of the duct and the weight of the vehicle when loaded ready for a trip, said vehicle being of selected unloaded weight and selected length such that, over the range of vehicle weights caused by varying I loading, a prepressure may be selected to enable operation of the system via opening the entrance valve for propulsion of the vehicle from the first station past the entrance valve into the duct, atmospheric pressure behind the vehicle acting to accelerate the vehicle through the duct, and the vehicle acting to compress air ahead of the vehicle, opening the exit valve when the pressure of air ahead of the vehicle generally reaches atmospheric pressure, the vehicle the coasting, closing the entrance valve behind the vehicle so that, as the vehicle travels forward, air behind the vehicle is expanded and its pressure drops, whereby the vehicle decelerates, bringing the vehicle to rest in the second station, and then closing the exit valve behind the vehicle with the pressure ahead of the vehicle rising to atmospheric pressure and the exit valve being opened before it is necessary to close the entrance valve for deceleration.

10. In a ground transportation system as set forth in claim 9, wherein the vehicle is of such selected unloaded weight and of such length that the prepressure may be selected to effect rise of pressure in front of the vehicle to atmospheric pressure before the air pressure behind the vehicle drops below atmospheric pressure. 

1. The method of operating a ground transportation system in which a vehicle is propelled as a free piston through a duct from a first station at one end of the duct to a second station at its other end, the duct having an entrance valve at said one end and an exit valve at its said other end adapted when closed to block off a section of the duct from valve-to-valve, each of said stations being in communication with the earth''s atmosphere, comprising starting with the vehicle in the first station, both valves closed, and said section evacuated to a pressure below atmospheric pressure; opening the entrance valve for propulsion of the vehicle from the first station past the entrance valve into the duct, atmospheric pressure behind the vehicle acting to accelerate the vehicle through the duct, and the vehicle acting to compress air ahead of the vehicle; opening the exit valve when the pressure of air ahead of the vehicle generally reaches atmospheric pressure, the vehicle then coasting; closing the entrance valve behind the vehicle so that, as the vehicle travels forward, air behind the vehicle is expanded and its pressure drops, whereby the vehicle decelerates, bringing the vehicle to rest in the second station; and then closing the exit valve behind the vehicle; and wherein the vehicle is weighted after having been loaded in the first station before opening the entrance valve; and the pressure in the evacuated duct is adjusted, in accordance with the weight of the vehicle and the length of the duct, to a value such as to provide the desired acceleration.
 2. In a ground transportation system having a duct through which a vehicle is propelled as a free piston from a first station at one end of the duct to a second station at the other end of the duct, the duct having an entrance valve at its said one end and an exit valve at its said other eNd adapted when closed to block off a section of the duct from valve-to-valve, each of said stations being in communication with the earth''s atmosphere, a manifold, a pump for evacuating the manifold, and a port having a pressure regulator valve therein interconnecting the manifold and the duct, said system having a plurality of stations greater than two with a duct between each two successive stations, each duct having an entrance valve at its said one end and an exit valve at the other, said manifold extending lengthwise of the system, there being a port with a pressure regulator valve therein interconnecting the manifold and each of the ducts.
 3. In a ground transportation system as set forth in claim 2, the length of the vehicle being less than the length of the duct between two successive stations.
 4. In a ground transportation system as set forth in claim 2, the length of the vehicle being greater than the length of the duct between stations.
 5. In a ground transportation system as set forth in claim 2, said system having two ducts extending side-by-side between each two successive stations, there being a port with a pressure regulator valve therein interconnecting the manifold and each of the two ducts between successive stations.
 6. In a ground transportation system having a duct through which a vehicle is propelled as a free piston from a first station at one end of the duct to a second station at the other end of the duct, the duct having an entrance valve at its said one end and an exit valve at its said other end adapted when closed to block off a section of the duct from valve-to-valve, each of said stations being in communication with the earth''s atmosphere, a manifold, a pump for evacuating the manifold, and a port having a pressure regulator valve therein interconnecting the manifold and the duct, said vehicle being a train which, adjacent its forward and rearward ends, has a sliding fit in the duct providing a seal which is sufficient to minimize leakage of air past its said ends, check valve means at the forward end of the train, and check valve means at the rearward end of the train, the forward check valve means permitting flow of air back to the train when the air pressure ahead of the train is atmospheric and the air pressure behind the train is less than atmospheric and checking the flow of air from behind the forward end of the train to the duct ahead of the train when the air pressure behind the train is atmospheric and the air pressure ahead of the train is less than atmospheric, the rearward check valve means permitting flow of air forward to the train from behind the train when the air pressure behind the train is atmospheric and the air pressure ahead of the train is less than atmospheric and checking the flow of air from ahead of the rearward end of the train to the duct behind the train when the air pressure ahead of the train is atmospheric and the air pressure behind the train is less than atmospheric.
 7. In a ground transportation system as set forth in claim 6, said train having forward and rearward end cars each having said sliding fit in the duct and passenger cars between the end cars, each check valve means comprising a passage through the respective end car and a check valve in said passage.
 8. In a ground transportation system as set forth in claim 6, said check valve means being constituted by unidirectional seals adjacent the ends of the train.
 9. In a ground transportation system having a duct through which a vehicle is propelled as a free piston from a first station at one end of the duct to a second station at the other end of the duct, the duct having an entrance valve at its said one end and an exit valve at its said other end adapted when closed to block off a section of the duct from valve-to-valve, each of said stations being in communication with the earth''s atmosphere, means for evacuating said section of the duct to a selected prepressure before each trip of the vehicle therethrough, said prepressure bEing selected in accordance with the length of the duct and the weight of the vehicle when loaded ready for a trip, said vehicle being of selected unloaded weight and selected length such that, over the range of vehicle weights caused by varying loading, a prepressure may be selected to enable operation of the system via opening the entrance valve for propulsion of the vehicle from the first station past the entrance valve into the duct, atmospheric pressure behind the vehicle acting to accelerate the vehicle through the duct, and the vehicle acting to compress air ahead of the vehicle, opening the exit valve when the pressure of air ahead of the vehicle generally reaches atmospheric pressure, the vehicle then coasting, closing the entrance valve behind the vehicle so that, as the vehicle travels forward, air behind the vehicle is expanded and its pressure drops, whereby the vehicle decelerates, bringing the vehicle to rest in the second station, and then closing the exit valve behind the vehicle with the pressure ahead of the vehicle rising to atmospheric pressure and the exit valve being opened before it is necessary to close the entrance valve for deceleration.
 10. In a ground transportation system as set forth in claim 9, wherein the vehicle is of such selected unloaded weight and of such length that the prepressure may be selected to effect rise of pressure in front of the vehicle to atmospheric pressure before the air pressure behind the vehicle drops below atmospheric pressure. 